This invention relates to a device for reciprocating a plunger or the like which converts rotation of an electric motor into reciprocation of a plunger or a piston (hereinafter any member which is reciprocated by the device will be referred to as "a plunger or the like" when applicable). The device is, for instance, built in an anti-lock brake system (ABS) in a large motor vehicle.
When the anti-lock brake system in a motor vehicle is in operation, a so-called "pumping brake" is automatically applied so that a wheel locking state and a wheel unlocking state are sensitively switched over to each other. Therefore, in this case, it is necessary to sensitively switch the relevant hydraulic pipe lines. In the case of an anti-lock brake system for large motor vehicles such as trucks and trailers, a mechanism as shown in FIGS. 3 and 4 is employed for switching the hydraulic pipe lines.
A drive shaft 2, which are supported by bearings 1 and 1 at both ends, is rotated by an electric motor 3 set between the bearings 1 and 1. A crank 4 is provided at one end (the left end in FIG. 3) of the drive shaft in such a manner that it is off-centered from the drive shaft 2 as much as .delta.. An outer race 5 is supported through needle 6 on the crank 4 in such manner that the crank 4 is rotatable relative to the outer race 5. In the case of FIG. 3, the crank 4 is cantilevered from the end portion of the drive shaft 2. However, it may be replaced with a center crank provided at the middle of the drive shaft 2 as shown in FIG. 4.
One end of a plunger 7 is held elastically abutted against the outer cylindrical surface of the outer race 5 through a spring means 71. More specifically, the plunger 7 is supported by a guide 72 in such a manner that it is allowed to displace only axially (vertically in FIG. 3 or 4), and it is elastically abutted against the outer cylindrical surface 5a of the outer race 5. The friction between the outer race 5 and one end of the plunger 7 prevents the outer race 5 from rotating relative to the plunger 7.
The mechanism is designed as described above. Hence, when the electric motor is switched on, the drive shaft 2 is rotated, so that the outer race 5 is whirled with an eccentricity of 6. This whirling motion causes the plunger 7 to reciprocate axially. As a result, the other end portion of the plunger 7 operates a control valve built in the anti-lock brake system (ABS); that is, a pumping braking operation is carried out.
The conventional device for reciprocating a plunger or the like which is built in an anti-lock brake system is designed and operated as described above. However, it should be noted that it is rather difficult to form the crank 4 which is off-centered from the drive shaft 2, which increases the manufacturing cost of the device as much. In addition, the device is disadvantageous in the following points: The plunger may give an impact load or unbalanced load to the outer race 5, thus cracking the latter 5, Furthermore, oil may leak through the plunger, as a result of which the grease provided inside a space between the outer race 5 and an inner race may flow out of it. These difficulties, in addition to the manufacturing cost, make it not practical to build the device in the anti-lock brake system of a small motor vehicle which is relatively low in manufacturing cost. In addition, the device is low in durability, and accordingly in reliability.
In reciprocating the piston of a hydraulic pump with the device, the performance of the pump may be improved by increasing the eccentricity .delta. of the crank 4 with respect to the drive shaft 2. However, the method is not practical as follows: In the case where the eccentricity .delta. of the crank in the above-described device is increased, it is essential to maintain the rigidity of the drive shaft. For this purpose, it is necessary to increase,for instance, the outside diameter of the latter 2. This means that the device is increased in weight as much.